The present invention relates to optoelectronic modules and in particular to an optical subassembly providing for EMI reduction.
High speed optoelectronic transmitters are employed in data communications systems wherein large amounts of data are to be transferred at high speeds. In such systems, optoelectronic transmitters convert binary data signals from electrical impulses carried by electrical conductors such as copper wire and circuit traces to optical signals that may be transmitted over optical media such as fiber optic cables.
Some optoelectronic modules, such as 1.times.9 transceiver modules, are configured to be mounted directly to a printed circuit board within the host device. In this arrangement contact pins extend from the module and are soldered directly to contact points on the printed circuit board. The module is usually mounted near the edge of the printed circuit board such that the optical end of the module will protrude through a slot in an adjacent metal faceplate that may be mounted to the metal chassis of the host device.
The transciever module must include provisions for connecting the module to an optical transfer medium such as a fiber optic cable. A typical arrangement common to 1.times.9 modules is to provide a transceiver module having an SC-duplex fiber optic connector receptacle integrally formed at the optical end of the module. The SC-duplex receptacle is configured to receive an SC-duplex connector to couple a pair of optical fibers to the optoelectronic module. A first optical fiber carries optical signals transmitted by the module, while a second carries optical signals to be received by the module.
Optoelectronic transceivers are often high speed devices capable of transmitting serial data streams at speeds above 1 gigabaud. At these high data rates, electronic components and circuitry within the module tend to radiate high frequency noise that can interfere with surrounding equipment. Therefore, care must be taken to prevent spurious emissions from escaping from the module housing and disrupting the operation of nearby devices.
A host device such as computer or a mass storage device will typically include a conductive chassis or case surrounding the internal electronic componentry. In most cases, the chassis will be connected to earth ground thereby establishing a reference potential on the conductive chassis known as case ground. The electronic circuitry for interfacing with the optoelectronic module will be completely enclosed within the grounded metallic chassis. However, an opening in the form of a rectangular cutout must be provided for allowing a fiber optic connector to be connected to a board mounted transceiver located within the chassis. Undesirably, the opening provided to receive the fiber optic connector provides an aperture through which high frequency electromagnetic interference (EMI) can escape the grounded chassis to interfere with the operation of surrounding equipment. Therefore, in order to reduce spurious emissions, it is important to limit the size of the emission aperture of the optoelectronic connector receiving slot formed in the host chassis.
FIG. 1 shows a portion of the optical end of a prior art optoelectronic transceiver module 10 inserted within the chassis 12 of a host device. The module 10 includes a metal or metallized connector clip comprising a first prong 14 and a second prong 52 for receiving and retaining a fiber optic connector. Aligned concentrically within the connector clip is a Transmitting Optical Sub-Assembly (TOSA). The TOSA includes a plastic housing 16, a focusing element 24, an annular mounting surface 32, an alignment ring 34, and an optical package 26. The external end of the TOSA housing 16 defines a ferrule receiving bore 18 configured to receive a fiber optic connector ferrule 20 which aligns the optical fiber 22 carried within the ferrule with the optoelectronic device contained within the optical package 26.
The optical package itself contains several discrete components, including a metal cover 28, a transparent window 29, and a conductive base or header 30. The header 30 and cover 28 are both formed of metal and are electrically connected such that the entire outer surface of the optical package, excluding the transparent window, is maintained at the same electrical potential. An insulating substrate 36 is provided within the optical package on the upper surface of the header. Electronic components including a Vertical Cavity Surface Emitting Laser (VCSEL) or high speed light emitting diode (LED) and associated contacts and power monitoring circuitry are mounted on the insulating substrate 36.
A plurality of signal pins extend through the header 30 and are connected to various components mounted on the insulating substrate 36. The signal pins shown in FIG. 1 include a laser bias signal pin 40, a monitor current signal pin 42, and a signal ground pin 44. The laser bias signal pin 40 and the monitor current signal pin 42 are insulated from the metal header 30 by glass sleeves 46 disposed between the pins and the header. The ground pin 44, however, is welded directly to header 30 such that the entire optical package is maintained at signal ground potential.
It should be noted that in the operation of optoelectronic modules it is necessary to separate signal ground from case ground. Therefore, in the module of FIG. 1 it is necessary to electrically isolate the outer metal covering of the optical package from the connecting clips 14, 15 which are maintained at case ground. In the device depicted in FIG. 1, several conductive components are in contact with the optical package 26, these include the alignment ring 34, the annular mounting surface 32 threaded onto the end of TOSA housing 16, and the metallized plastic transceiver housing 10. Together, these components form a continuous path to the signal ground pin 44 soldered to header 30, and therefore must be isolated from the case ground. TOSA 16 is mounted directly to the prongs 14, 52 of the metallized plastic connector clip which is integral to the metallized plastic transceiver housing 10. The transceiver housing 10 is connected to case ground via grounding tab 50. Because the TOSA housing 16 is connected to components held at both case ground and signal ground potential, the TOSA housing 16 itself must be formed of a non-conductive material in order to maintain isolation between the two distinct electrical ground potentials.
In the arrangement shown in FIG. 1, when the transceiver module is properly installed within the chassis of a host device, the emissions aperture of the SC connector receiving port of the transceiver module in the host chassis is defined by the outside diameter of the TOSA housing 16 and the distance between the grounded prongs 14, 52 of the connector clip. If an SC-Duplex connector is employed, prongs 14,52 are located 0.275" apart, and the outside diameter of the TOSA housing 16 and identified as AE, is approximately 0.250 inches.
The emissions aperture through a conductive surface is a function of both the dimensions of the cutout and the thickness of the panel. A relatively small hole in a large conductive surface will block low frequency signals as effectively as a solid sheet. At very high frequencies, however, the same small hole may allow a signal to pass the conductive barrier relatively unattenuated. Thus, as optoelectronic modules are operated at ever higher frequencies, it becomes necessary to take steps to reduce the emissions aperture of the modules in order to reduce the amount of EMI that can escape the module and interfere with surrounding equipment. Therefore, what is needed is an optoelectronic module which reduces the emissions aperture through which spurious electromagnetic emissions can escape the grounded host chassis.